Each time we go to Belize we use air miles, always used American Airlines and have had to pay departure.

This past visit we used air miles and the departure tax was included in those tickets.

We asked at the desk why we didn't have to pay. They didn't have an answer at that time. We had one leg of the flight to and from by TACA, otherwise we used American. Our Departure was TACA.
When using American Airlines exclusively, we have always had to pay the departure fee.

It must have been TACA that absorbed it this time.